The modern era of polyolefin catalysis has been dominated by a shift toward single-site catalysts, particularly metallocenes, and their ability to make polymers with differential, tailored properties. “Constrained geometry” complexes are one class of such single-site catalysts (see, e.g., U.S. Pat. No. 5,064,802). The complexes feature a cyclopentadienyl (“Cp”) or Cp-like ligand that coordinates to the transition metal (typically a Group 4 metal) in η5 fashion. The Cp ligand is bridged, usually to an anionic amido group that also bonds to the metal. Upon activation, the cationic center is highly exposed, which contributes to good activity and enables excellent incorporation of α-olefin comonomers (1-butene, 1-hexene, etc.) into the growing polymer chain. Thus, catalysts having constrained geometry are particularly valuable for making materials with low or very low densities, such as LLDPE and plastomers.
Considerable efforts (discussed by Joon T. Park et al., Organometallics (2000) 19 1269 and references cited therein) suggest some limitations of constrained geometry complexes. In particular, changing from tetramethylcyclo-pentadienyl to the more readily available Cp or indenyl ligand generally hurts both productivity and the ability to incorporate comonomers. Changing the bridging group from the usual single-atom bridge (e.g., a divalent methylene or dimethylsilylene group) to even a simple two-carbon bridge (ethylene) has also proved unfavorable. In addition, titanium complexes appear to be much preferred over zirconium or hafnium, at least for polymerizing ethylene.
Park et al. (supra) reported interesting results with bridged cyclopentadienyl-hydrazido titanium complexes, which include coordination to Ti from a tetramethylcyclopentadienyl group that is bridged to a 1,1-disubstituted hydrazido donor, which is monoanionic and coordinates in η2 fashion to the metal. The terminal dimethylamino group is a neutral donor that completes the three-membered chelate:
The complexes are moderately active for polymerizing ethylene in the presence of MAO or ionic borate activators. The need to synthesize the bridged Cp* ligand makes this approach challenging from an industrial perspective.
The industry would benefit from the availability of constrained geometry-like catalysts based on readily available, inexpensive cyclopentadienyl ligands. Because of the relatively poor track record of Cp complexes currently available, however, it remains a challenge to identify viable complexes of this type. Ideally, a family of complexes that share attributes of the most useful constrained geometry systems—high activity, good comonomer incorporation—could be discovered while avoiding the need to synthesize a Cp* precursor.